1. Field of Invention
The present invention relates to an apparatus and a method for sensing quasi-static (near-field) time-varying magnetic fields radiated by magnetic field generators, such as those employed in short range communication systems, distance measuring systems, and systems for detecting, monitoring, tracking, or determining the location, direction, position, or orientation of a remote object, either animate or inanimate, in relation to reference point, such as the transmitter or a wireless perimeter. More particularly, the invention relates to a non-multiplexed, single-output magnetic field antenna which requires only one signal amplifier for signal processing and which provides an omnidirectional magnetic field response when rotated about a principal axis.
2. Description of the Related Art
Systems employing a generator of a time-varying electromagnetic field of a particular carrier frequency positioned at a first location and a magnetic field receiver positioned at a second location remote from the first location but within the near-field radiation zone are known in the prior art. Such systems are used for determining the distance between the generator and receiver locations and for determining the coordinates of the receiver""s location with respect to the transmitter""s frame of reference. Examples of such applications include location and tracking of a lead vehicle with respect to a following vehicle, location of child relative a parent""s location, location of a diver relative to a home-base boat, location of an animate or inanimate object relative to a kiosk, mapping or digitization of two- or three-dimensional surfaces, monitoring of a probe inserted into the body, monitoring position of personnel, equipment, and tools in underground and underwater applications, and monitoring body movements for biomechanical control systems or for nonverbal communications means. Such systems are additionally used for determining position and orientation of the receiver""s frame of reference with respect to the generator""s frame of reference. Examples of such applications include the monitoring of position and orientation of actors on a stage or production set, monitoring position and orientation of an aircraft relative to a landing zone, monitoring position and orientation of military personnel and equipment relative to a command post, monitoring position and orientation of one object relative to a mating object, launching an aircraft ordinance along a pilot""s line of sight, and orientation sensing for generation of virtual reality computer graphics. Another application for such systems is a short range communication link where the radiated signal is detectable at short ranges, but undetectable at longer ranges to enhance security and reduce interference. Examples include such applications as kiosk installations which interrogate a customer""s xe2x80x9csmart cardxe2x80x9d for identification purposes. Finally, such systems are used to establish wireless boundaries relative to the generator""s frame of reference. Examples of such systems include systems for training a dog or other animal to stay either inside or outside a wireless boundary and for monitoring the movement of institutionalized persons to determine when they attempt to stray beyond the prescribed wireless boundary.
All of these prior art systems typically employ a one-, two-, or three-axis magnetic field generator radiating at a particular carrier frequency, typically in the extremely low frequency (ELF) or very low frequency (VLF) ranges. The receiver is typically equipped with a multiple axis array of mutually-orthogonal individual one-axis loop antennas consisting of a plurality of conductor turns wound on a ferrite core to enhance coupling with the magnetic field for increased receiver sensitivity. Each individual one-axis loop antenna is typically connected to a corresponding signal amplification and processing electronics channel in the receiver such that a two-axis receiver typically requires a two channel receiver and a three-axis receiver typically requires a three channel receiver. This multiplicity of receiver channels is a distinct disadvantage in those applications where miniaturization of the receiver size and power requirements are important considerations. There remains a need for an improved magnetic field receiving antenna providing multi-axis sensing, but not requiring separate signal amplification and processing channels for each axis of interest.
Therefore, it is an object of the present invention to provide an antenna for sensing a time-varying magnetic field of a particular carrier frequency radiated by a magnetic field generator unit, or a two-axis, single output magnetic field sensing antenna.
It is another object of the present invention to provide a two-axis, single output magnetic field sensing antenna wherein the amplitude of the sensed magnetic field is invariant as the antenna is rotated about an axis lying orthogonal to the antenna""s sensitive plane and passing though its center.
It is a further object of the present invention to provide a two-axis, single output magnetic field sensing antenna wherein only one signal must be amplified and otherwise processed to obtain information about the receiver location within a particular plane.
Yet another object of the present invention is to provide a two-axis, single output magnetic field sensing antenna wherein no signal combining is necessary to compute the projected magnetic field amplitude.
A still further object of the present invention is to provide a two-axis, single output magnetic field sensing antenna which can be combined with a standard one-axis loop antenna to provide information about the magnitude of the incident time-varying magnetic field in three dimensions and the orientation of the antenna""s frame of reference relative to vector direction of the incident magnetic field.
An additional object of the present invention is to provide a two-axis, single output magnetic field sensing antenna which can be used to obtain the magnetic field""s orthogonal components lying within the sensitive plane and lying along each axis of a particular antenna frame of reference.
It is also an object of the present invention is to provide a two-axis, single output magnetic field sensing antenna which accurately senses the magnetic field.
One more object of the present invention is to provide a two-axis, single output magnetic field sensing antenna which can be used in low power applications to provide accurate information about the location of a receiver relative to a transmitter.
Another object of the present invention is to provide a two-axis, single output magnetic field sensing antenna which can be used in applications where consistent and repeatable distance measurements between a transmitter and receiver are required.
The present invention is directed to a receiving antenna at one location for sensing and detecting the time-varying magnetic field of a particular carrier frequency radiated by an electromagnetic field generator at another location. Compared to the prior art one-axis loop antenna, the present invention provides two axes of sensitivity with no increase in antenna signal amplification or processing requirements. When compared to the prior art two-axis antenna comprised of two mutually-orthogonal one-axis loop antennas, the present invention provides an equivalent two-axis sensitivity but with reduced signal amplification and processing requirements.
The antenna is totally passive and provides a single electrical output signal having an amplitude which is proportionally related to the magnitude of the incident magnetic field and invariant as the antenna is rotated about an axis lying orthogonal to the antenna""s sensitive plane and passing through its center. The amplitude of the antenna""s single output signal is a direct measure of the amplitude of the incident magnetic field vector as projected onto the antenna""s sensitive plane. Therefore, the amplitude of the antenna""s single output signal is also a direct measure of the square root of the total combined power contained in the two orthogonal magnetic field components lying within the antenna""s sensitive plane. This method for sensing magnetic field amplitude in a particular plane is an improvement over the standard method of using two separate, mutually-orthogonal one-axis loop antennas in that only one signal must be amplified and otherwise processed, rather than two. Additionally, no signal combining is necessary to compute either the projected magnetic field amplitude because the information is carried in the amplitude of the single output signal or the total magnetic field power contained within the two components of the magnetic field which lie within the antenna""s sensitive plane. Where three-dimensional information is needed, the two-axis, single output magnetic field sensing output antenna of the present invention is combined with a one-axis loop antenna positioned orthogonal to the invention""s sensitive plane such that two output signals of the combination carries the same information about the magnitude of the incident time-varying magnetic field as a conventional three-axis antenna consisting of three separate and mutually orthogonal one-axis antennas. The combined antenna using the present invention is preferred over a standard three-axis antenna because only two signals must be amplified and otherwise processed, rather than three, to compute the total power in the incident magnetic field as commonly required in distance or proximity determining applications.
Furthermore, the phase difference between the phase of the time-varying incident magnetic field and the phase of the invention""s single output signal provides the information needed to obtain the magnetic field""s orthogonal components lying within the sensitive plane and lying along each axis of a particular antenna frame of reference. This method of sensing the magnetic field components lying in one of the planes of the antenna""s frame of reference is also an improvement over the standard method of using two separate, mutually-orthogonal one-axis loop antennas in that only one signal must be amplified and otherwise processed rather than two. The two-axis, single output antenna of the present invention is combined with a one-axis loop antenna positioned orthogonal to the invention""s sensitive plane such that the two output signals which are generated are processed to determine the magnetic field""s three spatial components with respect to the antenna""s frame of reference as is commonly required in systems that determine the orientation of the receiver frame of reference with respect to the generator frame of reference.
In the preferred embodiment, the antenna is constructed of identical inductors and standard value capacitors for ease of manufacture. The antenna is configured to have a particular bandwidth or quality factor, Q, as may be dictated by additional constraints. The antenna can be provided with a low-Q characteristic to avoid the need for adjustable components needed for trimming or fine-tuning during manufacture. Alternately, the antenna can be provided with a higher-Q characteristic for better rejection of out-of-band signals. In the preferred embodiment, the improved antenna is shielded for the purpose of attenuating interfering time-varying electric fields such as those radiating from the magnetic field generator unit. This shielding provides for improved magnetic field sensing accuracy and is conveniently provided by coating the antenna with a partially conducting coating of a particular resistivity such that the antenna""s electric field sensitivity is significantly attenuated with minimal attenuation of its magnetic field sensitivity.